Also called a 'thopter, it had adjustable wings and was jet-powered. Describes any plane capable of wing-beat flight in the manner of birds.

In Dune, science-fictional technologies were set at variance with feudal social systems. A technology which really brought out the fusion between these seemingly incompatible systems was an aircraft called an ornithopter, or 'thopter for short. The wing-beat movement was particularly effective on take-offs.

Paul fastened his safety harness, saw that his mother was secure, checked the aircraft. The wings were at full spread-rest, their delicate metal interleavings extended. He touched the retractor bar, watched the wings shorten for jet-boost takeoff the way Gurney Halleck had taught him. The starter switch moved easily. Dials on the instrument panel came alive as the jetpods were armed.

Although this quote gives primarily technical details about the aircraft, the winged motion was often used in association with the symbol of the hawk, which was part of the Atreides coat of arms. Here's another quote that describes its manner of flight:

He broke off as the Duke kicked on the jet brakes. The ship bucked as its tail pods whispered to silence. Stub wings elongated, cupped the air. The craft became a full 'thopter as the Duke banked it, holding the wings to a gentle beat, pointing with his left hand off to the east beyond the factory crawler.

Neither the word itself nor the concept were inventions of Herbert's. Da Vinci first described them (with very good engineering diagrams); the first flapping-wing working models were created by Alphonse Penaud in France in the late nineteenth century.

Here's a picture of a "flapwing flycar" from Arthur Radebaugh, the illustrator of the popular newspaper comic “Closer Than We Think”.
The Flapwing Flycar appeared in newspapers in 1958.